Characterization of immune responses induced by live attenuated simian immunodeficiency virus (SIV) strains may yield clues as to the nature of protective immunity induced by this vaccine approach. We investigated the ability of CD8+ T lymphocytes from rhesus macaques immunized with the live attenuated SIV strains SIV239?nef or SIV239?3 to inhibit SIV replication. CD8+ T lymphocytes from immunized animals were able to potently suppress SIV replication in autologous SIV-infected CD4+ T cells. Suppression of SIV replication by unstimulated CD8+ T cells required direct contact and was MHC-restricted. However, CD3-stimulated CD8+ T cells produced soluble factors that inhibited SIV replication in an MHC-unrestricted fashion by as much as 30-fold. Supernatants from stimulated CD8+ T cells were also able to inhibit replication of both CCR5- and CXCR4-dependent HIV-1 strains. Stimulation of CD8+ cells with cognate CTL epitopes also induced secretion of soluble factors able to inhibit SIV replication. Production of RANTES, MIP-1a or MIP-1a from stimulated CD8+ T cells of vaccinated animals was almost 10-fold higher than that from stimulated CD8+ T cells of control animals. However, addition of antibodies that neutralize these a-chemokines, either alone or in combination, only partly blocked inhibition of SIV and HIV replication by soluble factors produced by stimulated CD8+ T cells. Our results indicate that inhibition of SIV replication by CD8+ T cells from animals immunized with live attenuated SIV strains involves both MHC-restricted and unrestricted mechanisms and that MHC-unrestricted inhibition of SIV replication is due principally to soluble factors other than RANTES, MIP-1a and MIP-1a.